In the past, an organic electroluminescent element (hereafter, it is also called as an organic EL element) has been cited as an emission-type electronic display device.
An organic electroluminescent element has a constitution in which a light emitting layer containing a luminescent compound is interposed between a cathode and an anode. A hole injected from an anode and an electron injected from a cathode are made to be recombined in the light emitting layer to form an exciton. It uses emitted light (fluorescence and phosphorescence) when the above exciton is deactivated. An organic EL element enables to emit light at a voltage of several voltages to several ten voltages. Since it is a self-emission type, it has a favorable viewing angle and a high visibility. An organic EL element is a totally solid state element of thin-type. Consequently, it has been attracted attention from the viewpoint of space saving and portability. Further, it has also a feature of a surface light source.
In accordance with an increased expectation to an organic EL element as a surface emission and high efficient light source, it has been required to fulfill all of the needs of high efficiency, high luminance, and long life. In response to these requirements, it was disclosed an organic EL element having a multi-unit structure in which a plurality of light emitting units are laminated by connected in series through a charge generating layer. This is called as “a multi photon emission (MPE) technology” (refer to Patent document 1, for example).
An organic EL element using the MPE technology is provided with a plurality of light emitting layers and an intermediate layer between these light emitting layers for achieving long-life and high luminescence of the element. As an intermediate layer, there is a case in which a charge generating layer is formed. The charge generating layer is constituted by laminating with: an electron generating layer composed of an inorganic semiconductor material having an electron injection property; and a hole generating layer having an hole injection property. These electron generating layer and hole generating layer may be formed with vapor deposition of a single or plural materials.
Further, in recent years, it has been proposed a technology of forming an intermediate layer using a wet method in order to reduce a production cost of an organic EL element using the MPE technology. For example, it was proposed a method of forming an intermediate layer using a wet method by employing a curable material with heat or light, or an ionic polymer (refer to Patent documents 2 and 3, for example).
From paragraph [0290] of Patent document 2 and after, it is disclosed a need and a method of insolubilization treatment. In addition, in paragraph [0425] of Patent document 3, it is disclosed that when an upper layer is formed using a solvent on an underlayer that is adjacent to the upper layer, the material for the underlayer is selected to be hardly soluble in the solvent.
However, the present inventors found out the following. When an upper layer is formed in a position remoted from the adjacent position of an underlayer, a solvent used for the upper layer will penetrate in the underlayer to result in causing a problem of making damage. In order to resolve the problem, it is not sufficient to adjust insolubility between the two layers as described in Patent document 3. When the curing treatment to the layer is used as described in Patent document 2, the effect of the solvent used for the upper layer formed in a position remoted from the adjacent position may be reduced. However, an organic thin-film laminate or a substrate may cause damage, and a curing treatment device may be a large scale.